Nigerian police have dismissed any immediate threats from the Islamist group Boko Haram, which gave Christians in the north of the country with a three-day ultimatum to leave the area.

Police spokesman Yemi Ajayi, who called Boko Haram's claims "empty and calculated efforts to create fear in the people", said that security forces were working to protect all Nigerians.

Boko Haram issued the ultimatum and threatened to fight presidential troops after Nigerian President Goodluck Jonathan declared a state of emergency in regions of the country where the Salafist group has been active.

On Christmas Day, 49 people were killed in four attacks, most of them in a bombing at St Theresa's Catholic Church in the village of Madalla on the outskirts of the capital Abuja.

Boko Haram has accused the government of targeting Muslims.

"We find it pertinent to state that soldiers will only kill innocent Muslims in the local government areas where the state of emergency was declared," said Abdul Qaqa, spokesman for the group.

"We would confront them squarely to protect our brothers. We also wish to call on our fellow Muslims to come back to the north because we have evidence that they could be attacked. We are also giving a three-day ultimatum to the southerners living in the northern part of Nigeria to move away."

Ajayi dismissed Qaqa's threats and implored people to go about their business without fear or intimidation.

A state of emergency was declared in parts of the states of Borno, Niger, Pleageau and Yobe.

Nigeria's 160 million population is divided between a mainly Muslim north and Christian south.

President Jonathan said that the attacks threatened collective security and had "shaken the foundations of our corporate existence as a nation".